The invention concerns an electrolyte matrix, especially for a molten carbonate fuel cell. The electrolyte matrix consists of a matrix material that contains one or more lithium compounds, aluminum oxide, and a carbide. The invention also concerns a method for producing the electrolyte matrix.
To produce electric energy by means of fuel cells, a large number of fuel cells is usually arranged in the form of a stack. Each fuel cell has an anode, a cathode, and an electrolyte matrix arranged between them. The individual fuel cells are separated from each other by bipolar separators and are electrically contacted, and current collectors are provided at the anodes and the cathodes for electrically contacting them and for furnishing these electrodes with the fuel gas or the cathode gas. Sealing elements are provided at the edges of the anode, cathode, and electrolyte matrix, which form a lateral seal of the fuel cells and thus of the fuel cell stack to prevent anode and cathode material and electrolyte matrix material from escaping. The molten electrolyte fixed in the porous matrix typically consists of binary or ternary alkali metal carbonate melts. During operation, molten carbonate fuel cells typically reach operating temperatures of 600-650° C.
The matrix material of the electrolyte matrix has a number of jobs to perform. First of all, the matrix serves as a reservoir and substrate material for the electrolytes. A well-defined, highly porous structure of the matrix is a prerequisite for a high storage capacity. In addition, the matrix electrically insulates adjacent fuel half-cells and separates their gas compartments. Another requirement placed on the electrolyte matrix is that it must be capable of withstanding thermal tensile stresses induced by different coefficients of thermal expansion of the electrolyte matrix and the metallic components of the fuel cell and the fuel cell stack, especially the lateral sealing elements, that surround the electrolyte matrix. Tensile stresses of this type can cause cracking in the matrix, especially when the fuel cells are started up, and can thus lead to a reduction of power output and service life.
DE 100 60 052 A1 describes an electrolyte matrix, especially for a molten carbonate fuel cell, and a method for producing it, in which the matrix material contains one or more lithium compounds, aluminum oxide, and one or more zirconium compounds. When the fuel cell is started up, the matrix material undergoes a volume increase, which is intended to prevent cracking of the matrix due to different coefficients of thermal expansion of the matrix and the metallic components that surround it. This happens as a result of the synthesis of the matrix material when the fuel cell is started up, accompanied by an increase in volume. This previously known electrolyte matrix contains zirconium carbide to achieve a volume increase during the start-up of the fuel cell. A disadvantage of this previously known matrix is that it does not have the desired high strength either in the “green” state, i.e., before synthesis by the start-up of the fuel cell, or in the synthesized, i.e., sintered, state. Another disadvantage is that the zirconium carbide that is used is expensive and difficult to produce. Finally, the storage stability of this previously known matrix in the “green” state is limited.
DE 199 35 271 C2 describes a matrix material for a fuel cell, especially a molten carbonate fuel cell, which can be produced from a slurry filling by shaping and drying and contains one or more oxide ceramic powders, binders, plasticizers, and/or antifoaming agents. This previously known matrix material also contains homogeneously admixed oxidic secondary nanoparticles and is intended to offer high ductility and at the same time high strength.
DE 40 30 945 A1 describes a matrix material for a molten carbonate fuel cell, which contains a mixture of lithium aluminate and lithium zirconate.
Finally, U.S. Pat. No. 4,079,171 describes a molten carbonate fuel cell, in which the matrix contains principally lithium aluminate with a crystalline structure.